Localization of smooth muscle and nonmuscle actin isoforms in the human aqueous outflow pathway.

a-Smooth muscle actin is the isoform of actin restricted to vascular smooth muscle, pericytes, myofibroblasts and, certain other cells that are of myoid origin. We investigated the distribution of a-smooth muscle actin and nonmuscle specific filamentous actin in the human aqueous outflow system by immunohistochemical methods. Filamentous actin was observed in all cellular constituents of the outflow pathway, while distribution of a-smooth muscle actin was restricted to the ciliary muscle, to specific cells throughout the trabecular meshwork, and to cells adjacent to the outer wall and the collector channels. The ciliary muscle extended deep into the corneoscleral meshwork, far anterior to the scleral spur. These findings agree with our previous study localizing the distribution of smooth muscle myosin in the human aqueous outflow pathway. Although functionality of the immunoreactive cells needs to be demonstrated, our data show that a potentially contractile apparatus exists in a subpopulation of trabecular meshwork cells and in certain cells of the more distal components of the outflow system. Invest Ophthalmol Vis Sci 33:424-429,1992 Previous studies have demonstrated the presence of cells rich in actin in the human aqueous humor outflow pathway, specifically in the trabecular meshwork (TM) and adjacent to the outer wall of Schlemm's canal. 12 In addition, smooth muscle-like cells have been reported in the aqueous outflow system of the rat and rabbit. 3 " 6 We recently showed that cells containing smooth muscle-specific myosin are found within the TM, adjacent to the outer wall of Schlemm's canal and the collector channels. 7 Such structural and cytoplasmic contractile proteins are involved in many biological functions, and the presence of these proteins in cells of the outflow pathway could be important in aqueous humor outflow dynamics. The objectives of the present study were to determine, by immunohistochemical methods, whether smooth muscle-specific actin is present in the human outflow pathway and to compare the distribution of